Usually when you go faster you change drivetrains, like to RAGB's and straight axles or 700 peerless transmissions, the 600 series peerless are good for speed as well.
Even those should be modded for speed actually.
Bigger motors will net more speed , pulley changes usually will cure acceleration lag, but end up slowing you down, going with a go kart clutch would be quicker off the line also.
I ran a 6" motor pulley on my 18HP twin murray - it had a lil lag initially , but wound up quick enough.
Larger the motor pulley, slower the take off - even with a foot throttle.